This is a prospective, multi-center, open label, phase II study with a window of opportunity component in patients with recurrent squamous cell carcinoma of the head and neck (SCCHN) treated with adjuvant pembrolizumab following salvage surgery. Forty-five (45) patients will participate in this study to determine disease-free survival probability at 12 months. Secondary objectives include assessments of disease-free survival probability at 2 years, overall survival, adverse events and toxicity, and immune and molecular correlatives. For the window of opportunity component, patients will be randomized 3:1 in favor of receiving pembrolizumab 200 mg administered intravenously (IV) every three weeks for a maximum of two doses, versus placebo prior to salvage surgery. Approximately three to six weeks following the first dose of pembrolizumab, patients will undergo salvage surgery. Tumor tissue and blood will be collected at the time of surgery for immune correlative studies.